Compound semiconductor nanowire structures allow broad spectrum photonic devices to be implemented with a single growth process, thereby greatly increasing the efficiency of devices, exploiting them in solar cells and light emitting diodes (LEDs), for example, and reducing their costs. To date, solid state emitters exploiting compound semiconductor nanowires have been demonstrated in a range of materials for different wavelength ranges as depicted in Table 1. However, more recently, improved growth techniques, improved understanding of the effects affecting performance, the introduction of graded growth and self-organizing quantum structures have allowed multiple emitters to be integrated within the same nanowire to provide white LEDs as well as increasing emission efficiency. For example, InGaN nanowires have been demonstrated from the red-amber side of the spectrum through to the blue-violet side and near ultraviolet (UV) down to approximately 325-350 nm.
TABLE 1Examples of Semiconductor Alloys for LEDsWavelengthSemiconductor SystemMid-Infraredλ ≥1100 nm InGaAsPInfraredλ ≥760 nmGaAs, AlGaAsRed610 nm ≤ λ ≤ 760 nmAlGaAs, GaAsP, AlGaInP, GaPOrange590 nm ≤ λ ≤ 610 nmGaAsP, AlGaInP, GaP, InGaN,Yellow570 nm ≤ λ ≤ 590 nmGaAsP, AlGaInP, GaP, InGaNGreen500 nm ≤ λ ≤ 590 nmGaP, AlGaInP, AlGaP, InGaN,Blue450 nm ≤ λ ≤ 500 nmZinc selenide (ZnSe), InGaN,GaNViolet400 nm ≤ λ ≤ 450 nmInGaN, GaNUltravioletλ ≤400 nmBoron nitride (BN), AlN,AlGaN, AlGaInN
Despite the progress in electrically injected semiconductor lasers in the visible, infrared, and terahertz wavelength ranges it has remained difficult to realize electrically injected semiconductor lasers or efficient light emitting diodes (LEDs) within the rich deep ultraviolet (UV) spectrum 10-21. Bridging this deep UV gap would allow the replacement of conventional mercury lamps by efficient solid-state UV light sources for a broad range of applications, including water purification, disinfection, bio-chemical detection, medical diagnostics, and materials processing, to name a few. In this context, AlGaN-based materials, with a direct energy bandgap in the range of 3.4 eV to 6.1 eV, have been intensively studied.
However, whilst optically pumped AlGaN quantum well lasers UV-B and UV-C bands have been demonstrated these have had relatively high thresholds as a result of the properties of conventional AlGaN materials including the large bandgap and large effective mass for both electrons and holes. Reducing this can be achieved by modifying the density of states (DOS) using quantum-confined nanostructures, such as quantum dots and quantum wires. With the use of such low-dimensional nanostructures, semiconductor lasers with significantly enhanced gain and differential gain have demonstrated. Quantum dot-like nanoclusters can also be induced by phase separation where, for example, the presence of In-rich nanoclusters has been commonly observed in InGaN-based quantum well lasers; and the resulting carrier localization has been identified as one of the major factors contributing to the excellent performance of GaN-based quantum well lasers operating in the near-UV, blue, and blue-green spectral ranges. However, the relatively small lattice mismatch (a maximum of 3% between GaN and AlN), has to date prohibited the realization of electrically injected quantum dot lasers in the deep UV band.
Accordingly, it would be beneficial to establish the formation of self-organized Ga(Al)N quantum dots in the deep UV spectral range allowing low-dimensional quantum-confined nanostructures, such as quantum dots and quantum wires, to be achieved allowing deep UV semiconductor lasers with significantly enhanced gain and differential gain to be implemented.
GaN-based nanowire heterostructures have been intensively studied for applications in light emitting diodes (LEDs), lasers, solar cells and solar fuel devices. Recent studies have shown that the surface charge properties play a dominant role on the device performance such that for the commonly reported axial nanowire LED designs they exhibit very low output power as a result of the large surface recombination and resulting poor carrier injection efficiency. Radial variations of In/Ga distribution have been observed in InGaN/GaN dot/disk/well-in-a-wire heterostructures. However, such radial variations were found to be insufficient to suppress non-radiative surface recombination under electrical injection. In this regard, the use of a large bandgap AlGaN shell covering the surfaces of axial InGaN nanowire LED heterostructures has been explored and shown substantial promise in reducing surface recombination leading to improved carrier injection efficiency and output power. In these approaches, however, either relatively thick AlGaN layers were grown either on the top p-GaN region of the InGaN/GaN nanowires or incorporated within the device active regions, In each case the intention being to form an AlGaN shell for surface passivation. However, each approach leads to increased complexity in the device design, growth and fabrication processes thereby reducing yield/performance and increasing costs for devices. Moreover, a fundamental understanding of the impact of the AlGaN shell structure on the carrier dynamics and device performance has remained elusive.
Accordingly, it would be beneficial to provide designers of semiconductor nanowire emitting devices and their manufacturing operations with a means of implementing InGaN/AlGaN core-shell quaternary nanowire heterostructures wherein the In-rich core and Al-rich shell spontaneously form during the growth process. It would be further beneficial for these core-shell quaternary nanowire heterostructures to be tunable in emission wavelength across the visible spectral range allowing discrete high efficiency coloured nanowire LEDs, multi-colour high efficiency nanowire LEDs, and white high efficiency nanowire LEDs to be formed through adjustments in the growth parameters. Further, the inventors beneficially establish a direct correlation between the output power, carrier lifetime, and shell thickness to provide a robust, large bandgap shell structure methodology for dramatically enhancing the performance of axial nanowire LEDs for the solid state lighting and display applications.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.